Silica sols are useful materials in wide commercial use in many fields such as the fiber, paper, casting, ceramic, and catalyst industries. In particular, silica sols are very useful in the catalyst manufacturing industry as catalyst constituents or carriers. However, for some catalytic reactions, the presence of sodium and/or alumina (or aluminum) components in catalysts is undesirable in many instances. Furthermore, to ultrahigh-silicate zeolites (used as shape-selective catalysts and or adsorbents, either by themselves or in combination with other catalyst components), the presence of sodium is sometimes detestable, and, in extreme instances, those zeolites in which the Si/Al (or SiO.sub.2 /Al.sub.2 O.sub.3) ratio is infinite, namely, those absolutely free of alumina (or aluminum), are strongly desired. Regrettably, currently available silica sols are not so free of sodium and/or alumina (or aluminum) components as to meet all the above requirements, but contain, as impurities, sodium and alumina components (in certain instances, in unacceptable amounts). Therefore, establishment of a method for producing low-alkali metal, low-alumina, high-concentration silica sols at low cost would contribute not only to the catalyst manufacturing industry, but also various technical fields in which catalysts prepared using such silica sols are used.
Silica sols are colloidal suspensions in water of fine silica particles several millimicrons to several hundred millimicrons in primary or ultimate particle size. Commercially available silica sols are in most cases produced by using water glass (sodium silicate) as the starting material and subjecting this starting material to ion exchange, dialysis, ultrafiltration or gel peptization, for instance. Therefore, by any of these methods, the silica sols obtained contain typically certain amounts of sodium and alumina. Among commercial silica sols, some are claimed to be low-alkali metal and low-alumina; however, the sodium and alumina contents thereof are generally several hundred to several thousand and several hundred to ten and odd thousand ppm (parts per million parts), respectively, on the silica basis. As mentioned hereinabove, aqueous silica sols lower in sodium and/or alumina content are required for some purposes, but available commercial products do not satisfactorily meet requirements.
On the other hand, low-alkali metal, low-alumina silica powders are rather readily available on the market. It would be desirable to convert such silica powders into aqueous sols so as to obtain low-alkali metal, low-alumina silica sols. However, attempts to this end have failed so far.
For instance, several attempts to disperse fine silica powders obtained by hydrolytic combustion of silicon tetrachloride with a combustible gas (one of typical methods of producing high purity silica) in water are found in the prior art. However, these prior art techniques have all been unsatisfactory.
As one of the prior art techniques such as mentioned above, there is a proposal described in British Patent Specification No. 1,326,574 relative to the production of a dispersion of a fine silica powder relatively large in ultimate particle size and having good dispersibility. According to the proposal, silica particles of from 40 to 120 millimicrons in ultimate particle size are dispersed in water adjusted to a pH of at least 7 with an alkali metal hydroxide. However, this method allows contamination with alkali metals and moreover is not effectively applicable to silica powders of small particle size.
Other examples of the above-mentioned prior art techniques disclosing methods of forming such kinds of fine silica powders into an aqueous sol include U.S. Pat. Nos. 2,630,410 and 2,984,629. According to the former, the solation in water is effected in the presence of boric acid or an alkali metal borate in the aqueous medium, for the prevention of gelation of the resultant sol, whereas according to the latter, the solation in water is caused by the action of a mechanical shearing force (e.g., milling) in the presence of an alkali metal hydroxide (in an amount sufficient to make the pH of the aqueous sol 8.5 to 10.5) and a dispersing agent (arylsulfonic acid or alkylarylsulfonic acid). Both methods require addition of an auxiliary agent or agents for dispersion or stabilization to the water-silica powder mixture and such agents remain in the resulting sols as impurities. Therefore, these methods, too, fail to provide those silica sols that are very low in impurity content.
While the prior art techniques mentioned above use silica powders very high in purity, the use of an additive such as an alkali metal-containing substance as an auxiliary agent is unavoidable, because with such auxiliary agents it has been difficult to form said powders into aqueous sols. As a result, even so-called high purity silica powders are contaminated with additives as impurities.
There is also an example in which ultrasonic waves were applied in dispersing silicic acid gels. Experiments of this kind are reported in N. S. Bubyreva and B. P. Bindas, Colloid J. USSR (Engl.), Vol. 21, pp. 377-380 (1959). However, the starting material used in the experiments reported in the above-cited publication was not a fine silica powder as used in carrying out the present invention but was silicic acid precipitated from an acidic solution in a gel state. In addition, the ultrasonic waves used in said experiments for dispersing silicic acid gels to make up them into sols had considerably high frequencies of 1 to 8 megahertz (i.e., 1,000 to 8,000 kilohertz). As detailedly mentioned later herein, ultrasonic waves in such frequency range are not effective in the practice of the present invention. Moreover, the sols obtained in the Bubyreva et al. experiments had concentrations of only up to 5 g SiO.sub.2 /liter. Such concentrations are inadequately low for commercial use. For commerical application, silica sols should have a silica concentration of at least 15% by weight (about 150 g SiO.sub.2 /liter), and preferably from 20 to 30% by weight or higher. In these respects, the experiments described in the Bubyreva et al report cited above are quite distinct in object, constitution, and effects from the present invention, as will become clearer from the description below.